MYSQL: Insert column if not exists - php

Question

I am trying to add a column by checking if it exists. If not it should give message that it already exists with the code below.
$prefix = 'vm_';
$col_name = 'checking';
$col = "SELECT ".$col_name." FROM ".$prefix."users";
if (!$col){
$insert_col = "ALTER TABLE ".$table." ADD ".$col_name." DATETIME NOT NULL";
mysql_query($insert_col);
echo $col_name.' has been added to the database';
} else {
echo $col_name.' is already exists';
}
But it doesn't add any column and directly displays message that the column already exists.

Answer 1

You never execute your query, your condition is instead your query string:
if (!$col) { //always false, since a nonempty string is always truthy

Answer 2

Here is the final code. There was stupid mistake; I didn't use mysql_query for $col
$prefix = 'vm_';
$col_name = 'checking';
$col = mysql_query("SELECT ".$col_name." FROM ".$prefix."users");
if (!$col){
//$insert_col = "ALTER TABLE ".$table." ADD ".$col_name." DATETIME NOT NULL";
mysql_query("ALTER TABLE ".$prefix."users ADD ".$col_name." DATETIME NOT NULL");
echo $col_name.' has been added to the database';
} else {
echo $col_name.' is already exists';
}

Answer 3

You have to run your query before you check statements with it. It's like you can't see whether the box is empty of not (i.e. result of your query) without opening the box (i.e. running the query)
Replace your line 04 with this
$col = mysql_query("SELECT ".$col_name." FROM ".$prefix."users");
Then your problem will be solved
For more information on mysqli, please read this article
You might consider moving on to PDO statements as well.
Hope this helps

Answer 4

You haven't executed your query. First execute your query, then check the condition on it.

Answer 5

This is what I did and it works:
$db= new mysqli('host','user','password','db');
$query = "SHOW COLUMNS FROM tablename LIKE 'columnname'";
$db->query($query);
if(empty(empty($db->num_rows)) {
$alter = "ALTER TABLE tablename ADD columnname varchar(50) NOT NULL";
$db->query($alter);
} else {
echo 'column exists';
}